1. Field of the Invention
The present invention relates to a fusing system of an image forming apparatus such as a laser printer, a facsimile machine or a copier, and a temperature control method thereof. More particularly, the present invention relates to a fusing system of an image forming apparatus and a temperature control method thereof, which provide stable fusing of an image on a recording medium such as a paper by minimizing the temperature change of the surface of a fusing roller of a fusing unit of the image forming apparatus.
2. Description of the Related Art
A general electrophotographic image forming apparatus such as a copier or a laser printer prints images on a recording medium such as a paper sheet by a series of processes. These processes include an electrifying process which charges the surface of a photosensitive drum with a predetermined electric potential by rotating an electrostatic charging roller near the photosensitive drum. Other processes are a light exposure process which irradiates laser beams from a laser scanning unit (LSU) onto the surface of the photosensitive drum, thereby forming a desired electrostatic latent image and a developing process which develops the latent image on the photosensitive drum into a visible toner image by use of powder toner. Still another process is a transfer process which transfers the toner image of the photosensitive drum onto a paper sheet passing through the photosensitive drum and the transfer roller contacting the photosensitive drum with a predetermined pressure, by applying a predetermined transfer voltage to the photosensitive drum. Another process is a fusing process which fuses the toner image to the paper sheet by heating the paper sheet through a fusing unit which includes a fusing roller.
For the fusing process, a heater such as a halogen lamp is generally used, while being mounted inside the fusing roller and/or the fusing backup roller. Accordingly, the surface of the fusing roller is heated to a predetermined temperature by the radiant heat from the heater. Referring to FIG. 1, a fusing system 10 of an electrophotographic portion of a conventional image forming apparatus is described. A conventional fusing system 10 generally includes a cylindrical fusing roller 11 and a halogen lamp 12 mounted approximately at the center of the inside of the fusing roller 11. The halogen lamp 12 generates heat from within the fusing roller 11, and therefore, the fusing roller 11 is heated by the radiant heat from the halogen lamp 12.
A fusing backup roller 13 is mounted at the lower part of the fusing roller 11. As shown in FIG. 3, the fusing backup roller 13 is elastically supported by a spring device 13a to urge a paper sheet 14 passing between the fusing roller 11 and the fusing backup roller 13 toward the fusing roller 11.
Accordingly, a powder toner image 14a formed on the paper sheet 14 is pressed and heated by the pressure and heat while it is passed through the fusing roller 11 and the fusing backup roller 13. As a result, the powder toner image 14a is melted onto the paper sheet 14 by the heat and pressure applied from the fusing roller 11 and the fusing backup roller 13.
The fusing roller 11 has a thermistor 15 to detect the surface temperature of the fusing roller 11 in the form of an electric signal, a thermostat 16 to cut off a power supply to the halogen lamp 12 when the surface temperature of the fusing roller 11 exceeds a predetermined threshold, and a power switching part 19 (FIG. 1) such as a thyistor to switch a power supply of an AC power source 18 to the halogen lamp 12 in accordance with the signal received from a controller 20.
The thermistor 15 detects and transmits the surface temperature of the fusing roller 11 to the controller 20, and the controller 20 controls the power supply to the halogen lamp 12 through the power switching part 19 by comparing the detected temperature with a predetermined reference temperature, and thereby controls the surface temperature of the fusing roller 11 to a certain temperature suitable to fuse the image on the paper sheet 14.
The controller 20 generally uses a temperature control process which includes an initial heating stage in which the surface of the fusing roller 11 is heated to a print standby temperature (such as 165° C.), and a print standby stage in which the image forming apparatus waits for the print command while maintaining the surface temperature of the fusing roller 11 at the print standby temperature. The temperature control process also includes a print stage in which, with the input of a print command, the surface temperature of the fusing roller 11 is maintained higher than the print standby temperature in consideration of heat loss at the time of the fusing operation.
Referring to FIG. 4, in each of the temperature control stages, the controller 20 controls the power supply to the halogen lamp 12 to be on or off to maintain the surface temperature of the fusing roller 11 within a predetermined temperature range.
The on/off control by the controller 20 determines whether the current time ‘t’ has exceeded a predetermined temperature control period ‘t0’ (S1). If so, the controller 20 reads the surface temperature ‘T’ of the fusing roller 11, which is detected through the thermistor 15 and compares the read temperature ‘T’ with a predetermined target temperature ‘Tt’ in operation (S2). If the surface temperature ‘T’ is lower than the predetermined target temperature ‘Tt’, power is supplied to the halogen lamp 12 from the AC power source 18 to turn on the halogen lamp 12 (S3), and if the surface temperature ‘T’ is higher than the predetermined target temperature ‘Tt’, a power supply from the AC power source 18 is cut off so that the halogen lamp 12 is turned off (S4).
The thermostat 16 operates as an overheat prevention unit to protect neighboring components and the fusing roller 11 from unexpected temperature change when the temperature control by the thermistor 15 and the controller 20 fail s.
Because the conventional fusing system 10 controls the halogen lamp 12 by on/off control, the surface temperature of the fusing roller 11 is, irrespective of certain circumstances and conditions, controlled either by print standby temperature or in print temperature. This approach increases the surface temperature of the fusing roller 11 to the print standby temperature rapidly in the initial heating stage (S1). However, after reaching the print standby temperature, the surface temperature of the fusing roller 11 changes greatly and problems such as over-shoot are experienced. Additionally, power supply to the halogen lamp 12 is not optimized, and power consumption thereby increases.
When the surface temperature of the fusing roller 11 changes greatly, print temperature cannot be controlled in stable manner. As a result, image fusing on the paper becomes unstable.
FIG. 5 shows proportional integral derivative (PID) control in the respective temperature control stages intended to overcome the above-mentioned problems. The PID control keeps the surface temperature of the fusing roller 11 within a predetermined temperature range by controlling the power supply to the halogen lamp 12.
In the PID control, the controller 20 determines whether the current time ‘t’ has exceeded the predetermined temperature control period ‘t0’ (S1′). If so, the controller reads out the surface temperature ‘T’ of the fusing roller 11, which is detected through the thermistor 15 and compares the read temperature with a predetermined target temperature ‘Tt’ (S2′). If the detected surface temperature ‘T’ is lower than the target temperature ‘Tt’, the controller calculates a lamp “on time” in accordance with the equation (1) below. In operation S3′, the controller turns on the halogen lamp 12 by supplying power to the halogen lamp 12 from the AC power source 18 (S4′ and S5′). If the detected surface temperature ‘T’ is higher than the target temperature ‘Tt’, the controller turns off the halogen lamp 12 (S6′).On time(ms)=t0xα0x(Tt−T)/100  (1)where, α0 is a proportional coefficient.
The PID control is advantageous as compared to the on/off control when considering a stable temperature control with little over shoot. However, according to the PID control, the temperature control period ‘t0’ must be shorter.
Meanwhile, the fusing roller 11 of the conventional fusing system 10 is generally formed as an aluminum cylinder which has a rubber layer 11a of lower heat conductivity formed around the outer surface (see FIG. 2).
The rubber layer 11a keeps a uniform contact area between the fusing roller 11 and the fusing backup roller 13 (a so-called ‘nip area’) when the paper sheet 14 is passed therethrough such that a sufficient time is given for the heat transfer to the paper sheet 14. Additionally, the rubber layer 11a keeps the heat supplied from the halogen lamp 12 such that the surface temperature of the fusing roller 11 is prevented from dropping abruptly when the paper sheet 14 is passed therethrough. However, due to its low heat conductivity, the rubber layer 11a of the fusing roller 11 requires more time for the heat transfer from the halogen lamp 12.
To describe the above in more detail, FIG. 6 shows one example of temperature distribution of the fusing roller 11 measured upon activation of the halogen lamp 12 and varying with time. As shown in FIG. 6, temperature does not vary much along the entire thickness of the aluminum cylinder, but toward the outer surface where the rubber layer 11a is located, temperature decreases due to low heat conductivity.
More specifically, approximately 90 seconds after the switch-on of the halogen lamp 12, the temperature of the aluminum cylinder of the fusing roller 11 stays at approximately 230° C. At this time, the rubber layer 11a has the temperature at approximately 180° C., which is 50° C. lower than the temperature of the aluminum cylinder. Accordingly, if the halogen lamp 12 is turned off when the surface temperature of the rubber layer 11a of the fusing roller 11 reaches the fusing temperature such as 180° C., the surface temperature of the rubber layer 11a rises beyond the fusing temperature due to the temperature of the aluminum cylinder, which has already been heated to 230° C. On the contrary, if the halogen lamp 12 is turned on when the surface temperature of the aluminum cylinder of the fusing roller 11, which is further increased over the fusing temperature due to the heat of the aluminum cylinder of the fusing roller 11, is cooled below the fusing temperature, the surface temperature of the rubber layer 11a keeps dropping until the heat applied to the aluminum cylinder of the fusing roller 11 is transmitted to the surface of the rubber layer 11a. 
As described above, while the fusing roller 11 having the rubber layer 11a does not suffer abrupt temperature change due to the low heat conductivity of the rubber layer 11a, due to a rather great difference in conductivity between the rubber layer 11a and the aluminum cylinder, the temperature gap therebetween grows as the halogen lamp 12 is frequently turned on and off, or driven for a long period of time.
Additionally, considering that the paper sheet absorbs the heat of the surface of the fusing roller 11 when the paper sheet is fed to the fusing roller 11 of the fusing system 10, the temperature gap between the aluminum cylinder and the rubber layer 11a is widened.
As the temperature gap between the aluminum cylinder and the rubber layer 11a grows, the change in the surface temperature of the fusing roller 11 increases, subsequently causing an overshoot phenomenon. As a result, the fusing temperature cannot be controlled stably, and the image is unstably fused onto the paper sheet.